thumb|300px|Group of [[#Spinner and Fraser's dolphins|Fraser's dolphins]]

Cetacean bycatch (or cetacean by-catch) is the accidental capture of non-target cetaceans such as dolphins, porpoises, and whales by commercial fisheries. Bycatch can be caused by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets.

Cetacean bycatch is increasing in intensity and frequency. This trend is likely to continue due to the pressures of human population growth on the environment, the growing popularity of seafood, and the commercialization of deep-sea fishing as operations expand into waters which were previously protected. These commercial fishing vessels come into both direct and indirect contact with cetaceans. An example of direct contact is the physical contact of cetaceans with fishing nets. Indirect contact occurs through marine food chains, which are destabilized by the commercial fishermen severely reducing local fish populations that cetaceans rely on for food. In commercial fishing practices, cetaceans are captured as bycatch but then retained because of their value as food or bait.

right|thumb|A [[Dall's porpoise caught in a fishing net]]

Generally, cetacean bycatch is increasing. Most of the world's cetacean bycatch occurs in gillnet fisheries. They are fatal for small toothed whales (Odontocetes) and sperm whales, as well as other marine vertebrates such as sharks, sea birds and sea turtles. Many fisheries routinely use driftnets exceeding the EU size limit of 2.5 km/boat. This illegal drift-netting is a major issue, especially in important feeding and breeding grounds for cetaceans.

The tuna industry has achieved successes in reversing cetacean bycatch trends. International recognition of the problem of cetacean bycatch in tuna fishing led to the Agreement on the International Dolphin Conservation Program in 1999 and overall there has been a dramatic reduction in death rates. In particular, dolphin bycatch in tuna fishing in the East Tropical Pacific has dropped from 500,000 per year in 1970 to 100,000 per year in 1990, to 3,000 per year in 1999, and to 1,000 per year in 2006.

A shark culling program in Queensland, which has killed roughly 50,000 sharks since 1962, has also killed thousands of dolphins as bycatch. Queensland's "shark control" program has killed more than 1,000 dolphins in recent years, and at least 32 dolphins have been killed in Queensland since 2014. A shark culling program in KwaZulu-Natal, South Africa has killed at least 2,310 dolphins.

Cetaceans at risk

Bycatch is recognized as a primary threat to all cetaceans. The following cetaceans are at high risk for entanglement in gillnets:

Atlantic humpback dolphins

The Atlantic humpback dolphin (Sousa teuszii) is endemic to West Africa. Several stocks have been identified with numbers ranging from tens to a few hundred. Abundance estimates are lacking. Gaps in the species range and hence distribution are evident. Bycatch is only documented in a few West African countries. Surveys and evaluations need to be conducted to determine the presence/absence of humpback dolphins in their historical range. Conservation measures need to be implemented to save this species. Because many people live off the sea, it is not feasible to have complete gillnet closures. Some areas may be designated as off-limits to gillnet fisheries. Eco-tourism may be implemented successfully because of high species diversity. Analyses show that 50-70% of Gulf of Maine humpback whales (Megaptera novaeangliae) and North Atlantic right whale (Eubalaena glacialis) have been entangled at least once in their lifetime. Minke whales (Balaenoptera acutorostrata) are also at risk.

Burmeister's porpoises

The Burmeister's porpoise (Phocoena spinipinnis) is one of three cetaceans that are most often bycaught in Peru and Chile. Often, the harbour porpoise (Phocoena phocoena) is killed by incidental bycatch (10, 11, 12). Gillnets pose a serious threat to the harbour porpoise as they are extremely susceptible to entanglement. A study by Caswell et al. in the western North Atlantic combined the mean annual rate of increase of the harbour porpoise with the uncertainty of incidental mortality and population size. It was found that the incidental mortality exceeds critical values, thus showing bycatch is a significant threat to the harbour porpoise. In 2001, 80 harbour porpoises were killed in salmon gillnet fisheries in British Columbia, Canada.

Hector's and Maui's dolphins

thumb|Hector's dolphins have a unique rounded dorsal fin.

In New Zealand, these dolphins have a high rate of entanglement. Hector's dolphin (Cephalorhynchus hectori) is endemic to the coastal waters of New Zealand and there are about 7,400 in abundance. The Yangtze River finless porpoise (Neophocaena phocaenoides asiaeorientalis) also lives in the Yangtze River. Abundance has declined; there are fewer than 2000 porpoises left. As of 2024, there are less than 8 individuals left in the Gulf of California, making the species functionally extinct similar to the baiji.

Mitigating bycatch

Acoustic deterrent devices

The use of acoustic alarms to mitigate by-catch and to protect aquaculture sites has been proposed. Acoustic deterrent devices, or pingers, have reduced the number of cetaceans caught in gill nets. Harbour porpoises have been effectively excluded from bottom-set gill nets during many experiments for instance in the Gulf of Maine, along the Olympic Peninsula, in the Bay of Fundy, and in the North Sea. All of these studies show up to a 90% decrease in harbour porpoise bycatch. Pingers work by producing a sound that is aversive to the cetaceans (20; 15). There has been a recent re-evaluation of the potential benefit of pingers, and their use in other fisheries has expaneded due to their growing success.

An experiment on the California drift gill net fishery demonstrated how acoustic pingers reduce marine mammal bycatch. It showed that bycatch was significantly reduced for common dolphins and sea lions. Bycatch rates were also lower for other cetacean species, such as the Northern right whale dolphin, Pacific white-sided dolphin, Risso's dolphin, and Dall's porpoise. It is agreed upon that the more pingers on a net, the less bycatch. There was a 12-fold decrease in common dolphin entanglement using a net with 40 pingers. Cetaceans, extremely sensitive to noise, are effectively driven from their preferred coastal habitats by the use of these acoustic devices. In poorer quality habitats, harbour porpoises are subjected to increased competition for resources. This situation is recognized as range contraction, which can be a result of climate change, anthropogenic activity, or population decline. Large-scale range contractions are considered indicative of impending extinction. A similar form of deterrent is noise pollution originating from vessel traffic.

Barium sulfate

A promising gillnet that is effective in reducing bycatch for harbor porpoises contains barium sulfate. These nets are detected at a greater distance than conventional nets because the barium sulfate reflects the cetaceans' echolocation signal, thus rendering the nets more detectable by these animals. Barium sulfate makes the nets stiffer if it is added at high concentration. All three factors - echo reflectivity, stiffness, and visibility - may be important in reducing bycatch. Echo reflectivity appears to depend on particle size, not density. The denser iron oxide produced no reduction in bycatch. Fish takes in the Bay of Fundy were normal, except for haddock takes, which were down by 3-5%. The advantage of this approach is that it is passive and thus does not require batteries, and there is no "dinner bell" effect. The potential advantage of these nets is greatest in the artisanal fishery. NOAA is planning further testing to verify the effectiveness of the nets.

Fishing regulations and management

Management and regulation are severely lacking in many fisheries today. Management measures are urgently needed to monitor fisheries (and illegal fisheries) to protect cetaceans. Efforts to document bycatch should focus on gill-net fisheries, because cetaceans are more likely to be caught in gill-nets. Conservation efforts should be directed to areas where marine mammal bycatch is high but where no infrastructure exists to assess the impact.

Observers on boat

Another mitigation measure is to use observers on fishing vessels to spot cetaceans in the water in order to avoid bycatch.

In the U.S.

Some programs like Earth Island Institute's Dolphin Safe Label certification claim to require certification from onboard observers. However, the only fishery in the world where independent scientific observers certify whether a dolphin has been harmed is the Eastern Tropical Pacific, home to the AIDCP Treaty program. For all other tuna fisheries of the world, the efficacy of onboard observer certification has come under increasing scrutiny as such programs have proven indefensible or unmanageable:

In an interview with Radio Australia last year, Mark Palmer of EII confirmed that it is mostly the case that EII monitors do not go on board of the vessels, and their organization does not have the kind of resources to put observers on the "many thousands" of ships that are out there catching tuna.

Additionally, environmental groups have criticized Earth Island Institute's support of U.S. policies that do not require independent, on-board observation and instead only rely on self-certification by fishing captains. Even if there is a requirement of independent observers, the lack of uniformity in tracing and verifying certifications in different countries implies that non-certified products can still become certified if they are taken to a different port.

Other ways of mitigating bycatch

  • Implementing gear technology (changes in fishing gear and practices) was documented to mitigate cetacean bycatch.
  • Buying tuna and other seafood that has a dolphin safe label.
  • Buying Sustainable seafood.
  • Supporting sustainable seafood companies and restaurants.
  • Raising international awareness to assess, monitor, and mitigate bycatch problems.
  • Creating legislation on responsible fishing practices.